theory of flanking most advanced pawn, engine advises it but it doesn't seem so beneficial

I have a question about the theory of flanking the most advanced pawn.  The, engine advises it but it doesn't seem so beneficial as i'd just lose the pawn.

Move 3....Kd7  vs  3....c5

But granted nothing I did turned out to be beneficial either. Though I figured Kd7 seems better than leaving the knight in the starting position

I think too

The idea of such moves:

1) gain space

2) create tension

3) maybe get the upper-hand in the center

Ad 1) your C pawn is blocking you from developing the Queen and connecting the rooks. Furthermore, a c5 pawn controls d4

Ad 2) with creating tension you give your opponent a harder time, because taking on c5 might be disadvantagious

Ad 3) if white decides to take c5, then you might play e5 far easier, because the d4 is gone. Apart from that there is general rule in chess: "with pawns take towards the center"